Method for calibration of indirectly measured quantities

ABSTRACT

A method for calibrating a system in an oilfield. The method involves operating a component which includes both a direct sensor and an indirect sensor. Additionally, a processing-capable calibration unit in installed on the system and receives signals from the direct sensor and the indirect sensor. After the indirect sensor calculates indirect physical measurements and the direct sensor calculates direct physical measurements, the calibration unit compares the indirect physical measurements with the direct measurements and determines the difference between them. The calibration unit then computes a calibration command and communicates it to the indirect sensor, which uses this command to adjust its future indirect measurements.

STATEMENT OF FIELD

The invention relates to the control of performance quantities in oil operations such as drilling and casing. In particular, the invention relates to obtaining accurate and frequent samples of significant parameters such as torque applied to the equipment, mud flow, and pressure. In further particularity, the invention relates to correcting errors in indirect measurements caused by damping and similar intervening factors.

BACKGROUND

To extract oil from beneath the surface, excavators use complex oilfield operations dependent on teams of people and complex technology. For example, extracting oil requires an excavator to drill into the earth using a combination of equipment called a “drill string,” which enables a large drill bit to burrow into the surface. Another example of technology used in oil excavation is the hook, which supports heavy loads of equipment and material delivered into a wellbore and extracted from it.

In these operations, several physical quantities affect the methodology of an operation in progress. Such quantities include the torque sustained by a drill string, load applied to a hook, pressure, mud flow, and related measurements over time. These quantities may be estimated by using a mathematical derivation from the performance of mechanical equipment. For example, a user may estimate torque by determining the power or current draw of an electric motor.

Indirect determination of physical quantities offers a high sampling rate because the relevant data can be collected continuously. However, damping and similar error resulting from inefficiency, friction, imperfections in mechanical linkages, and displacement between the measuring instrument and the point of measurement may impair the accuracy of these measurements.

Recent technology offers a second approach through direct measurement of the same relevant quantities. Wireless sensors disposed on the equipment can directly measure the parameters, such as hook load and torque. To operate, these sensors use an independent power source. The addition of this power source causes the wireless sensors to take measurements at significantly lower frequencies, causing accurate measurements with low sample sizes. This characteristic creates a significant trade-off for persons engaged in energy operations.

SUMMARY

In accordance with preferred embodiments of the invention, there is provided a method for the dynamic calibration and error correction of indirect measurements through use of corresponding direct measurements. This dynamic calibration process requires two measuring devices, typically sensors, is applied continuously throughout a drilling or casing operation, and accounts for real time changes in the equipment's environment.

Through two or more sensors installed on drilling or casing equipment, the process allows a user to collect large samples of accurate data during an operation. To function, the calibration process requires simultaneous direct and indirect measurements of a physical quantity. In addition, a calibration device containing processing hardware, such as a microprocessor, enables performance of calibration algorithms.

The technique of indirect measurement is a process by which parameters relevant to drilling such as torque, mud flow, pressure, etc. are through calculating related aspects of the equipment's performance. For indirect measurements, a sensor is physically connected (typically hard-wired) to the equipment's machinery and detects operation data, such as current draw and power use by an electric motor. The sensor, upon reading the data, estimates the desired parameters, such as torque, by calculating their value through predetermined formulas and algorithms. This technique can generate large samples of data in a short time because the variables necessary for estimation are available within the equipment. However, mechanical imperfections such as equipment inefficiency, friction, and displacement from the point of measurement affect the sensor's estimates. This creates a significant damping effect that impairs the quality of estimates obtained by indirect measurement.

In comparison, direct measurement of the desired parameters typically offers a higher accuracy of measurement but lower sample sizes. Direct measurement requires the installation on subject equipment of a sensor capable of directly measuring a quantity or parameter. This process of direct measurement allows fewer intervening variables and factors to impair the quality of measurement. However, these direct measurement sensors generally require an independent power source and are thus incapable of generating as large of data sets as an indirect measurement sensor. This limitation causes difficulty in adjusting an operation dynamically even if the data provided is accurate.

Adding a calibration unit offers the ability to improve the accuracy of indirect measurements while preserving its large sample sizes. To achieve this result, the calibration unit is preferably connected to both an indirect sensor and a direct sensor. The calibration unit collects data from both the direct and indirect data sensors and compares the results of each. Using the submitted data, the calibration unit adjusts the indirect measurement sensor's estimation process to account for offsets and damping present in the previous recording. These calibrations are used to estimate the next set of data. This process is repeated continuously to enable dynamic adjustment of equipment during an operation.

For greater predictability and ease of use, the calibration cycle may be defined over specific time intervals. A sample of indirect measurements may be mathematically averaged over the corresponding time interval for which the direct sensor records a measurement. Averaging the indirect sensor's data sample data ensures that direct data for the same interval is comparable, improving the calibration's accuracy.

Further measures may improve the calibration process. The user may set an error limit based on the expected conditions of the drilling or casing operation. If the measurements obtained by the direct sensor exceed this error limit when compared to the indirect measurements and the direct sensor's previous measurements, the direct sensor's erroneous readings are discarded and the calibration process repeats. This added step prevents incorrect or outlier data from entering the calibration process and improves the accuracy of future readings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed understanding of the invention, reference is made to the accompanying wherein:

FIG. 1 is a block diagram of equipment used in a preferred embodiment of the invention;

FIG. 2 is a flowchart illustrating a first embodiment of the present calibration process;

FIG. 3 is a flowchart displaying additional steps for matching the time interval for a direct measurement sample with the time interval for an indirect measurement sample; and,

FIG. 4 is a modified flowchart displaying optional steps that correct for errors that would otherwise decrease accuracy in the calibration process.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a structural diagram of a preferred embodiment of the present calibration system includes a calibration system 10 and an associated environment 60. System 10 may be any device useful in an oilfield operation but may include, for example, items such as a drill assembly, drill bit, casing, pump, or similar device. A user of system 10 desires to obtain a large sample of accurate measurements of a parameter concerning the interaction of equipment 10 with its environment 60. The parameter may include any relevant measurement obtainable by direct measurement or derivation from the performance of equipment 10. Example parameters include torque, mud flow, pressure, drill string tension, and velocity.

To measure quantities, equipment 10 includes one or more indirect sensors 20. Indirect sensor 20 may be disposed within system 10 or housed within a discrete assembly, such as an instrumented sensor subsystem 22. System 10 also includes one or more direct sensors 30 installed nearby environment 60 at locations where the parameter may be measured or detected. Direct sensor 30 may be powered by a corresponding power supply unit 32.

System 10 also contains a calibration unit 40 which may send electrical signals to or receive indirect sensor data 24 from indirect sensor 20, and receives direct sensor data 34 from direct sensor 30. To calibrate data, calibration unit 40 requires processing hardware (not shown) such as a microprocessor or computer chip. Indirect sensor data 24 is an estimate of a parameter determined through allowing the processing hardware to compute it from performance variables, such power used or electric current drawn by a motor, or similar aspects of system 10. Direct sensor data 34 measures the parameter directly by direct sensor 30 but generates lower sample sizes than indirect sensor 20 because direct sensor 30 operates at a lower sampling rate due to battery 32.

Calibration unit 40, upon receiving data sets from indirect sensor 20 and direct sensor 30 for a parameter, determines a difference between the indirectly measured data 24 and the directly measured data 34. When the sample size of indirect data 24 exceeds the sample size of direct data 34, calibration unit 40 may compute a numerical average of indirect data 24 over the time necessary to collect direct data 34.

Calibration unit 40, after determining a difference between indirectly measured data 24 and directly measured data 34, provides sensor 20 or instrumented sensor subsystem 22 by electrical signal a calibration adjustment to use when calculating the next sample of data. Indirect sensor 20 then provides a new set of indirect data 24 which is in turn compared to next sample of direct data 34 obtained by direct sensor 30.

For improved performance, the user may set an error limit within calibration unit 40. After calibration unit 40 collects data from indirect sensor 20 and direct sensor 30, the difference between indirect sensor data 24 and direct sensor data 34 is compared the against error limit. If direct sensor 30 has obtained erroneous or outlying data due to unforeseen external factors, error limit 50 detects this problem and stops calibration unit 40 from using this data to calibrate indirect sensor 20. Instead, sensor 20 proceeds to the next sample of indirect data 24 without calibration and calibration unit 40 will attempt calibration in the next cycle. If the error limit is not exceeded, calibration unit 40 will signal indirect sensor 20 or instrumented sensor subsystem 24 for calibration.

Turning to FIG. 2, the process for calibrating indirect sensor 20 or instrumented sensor subsystem 22 will now be described. This process preferably occurs dynamically as during the operation 100 of equipment 10. While the operation 100 occurs, direct sensor 30 performs step 106 of collecting one or more measurements of a parameter. Simultaneously, instrumented sensor subsystem 22 or indirect sensor 20 performs step 102 of reading internal variables in equipment 10 and using them to estimate 104 the value of a parameter.

Next, calibration unit 40 receives the data samples obtained by indirect sensor 20 and direct sensor 30 and compares 108 the data for calibration. Based on this comparison, calibration unit 40 calibrates 110 instrumented sensor subsystem 22 or indirect sensor 20. This calibration is corrects the next collection 102 of indirect data 24, after which another calibration cycle preferably occurs.

FIG. 3 displays an optional additional step that may be applicable when the size of indirect data sample 24 exceeds direct data sample 34. In this instance, calibration unit 40 performs additional step 200 of averaging indirect data sample 24 over a predetermined time period. After collection of the sample of direct data 34 and indirect data 24, but before calibration of indirect sensor 20 or instrumented sensor subsystem 22. This additional averaging step 200 allows direct data sample 34 and indirect data sample 24 to reflect measurements taken over the same time period.

FIG. 4 displays additional optional steps in the calibration process to prevent erroneous calibrations. Calibration unit 40 may compare 302 indirect data sample 24 and direct data sample 34. Calibration unit 40 may also perform an additional step 300 of setting an error limit. If the disparity between the data exceeds error limit 50, calibration unit 40 signals indirect data sensor 20 or instrumented sensor subsystem 22 to collect another data set 24 without a modification through calibration. This prevents outlying data or intervening factors from incorrectly modifying future data collection. However, if the disparity does not exceed error limit 50, the calibration is permitted 304 to continue, improving the accuracy of future indirect data 24 collected by equipment 10.

Although the preferred embodiments of the present invention have been described herein, the above description is merely illustrative. Further modification of the invention herein disclosed will occur to those skilled in the respective arts and all such modifications are deemed to be within the scope of the invention as described by the appended claims. 

1. A method for calibrating an oilfield system, the method comprising: a) operating a component having a direct sensor, an indirect sensor, and a calibration unit wherein said calibration unit includes a processor; b) determining at least one indirect physical measurement with said indirect sensor; c) measuring at least one direct physical measurement with said direct sensor; d) comparing with said calibration unit said at least one indirect physical measurement with said at least one direct physical measurement, and; e) sending a calibration command from said calibration unit to said indirect sensor.
 2. The sensor calibration method of claim 1, further comprising the additional steps of embedding an error limit in said calibration unit and preventing said sending step if the difference between said direct measurement and said indirect measurement exceeds said error limit.
 3. The sensor calibration method of claim 1 wherein step b) further comprises averaging said at least one indirect physical measurement over a time interval.
 4. The sensor calibration method of claim 3 wherein step c) further comprises defining said at least one direct physical measurement within said time interval.
 5. The sensor calibration method of claim 1 wherein said indirect sensor receives said calibration command and adjusts future measurements based on said calibration command.
 6. A dynamic calibration apparatus comprising: an indirect sensor wherein said indirect sensor obtains a secondary indirect measurement and converts said secondary indirect measurement into a primary indirect measurement of a quantity; a direct sensor disposed on said subsystem wherein said direct sensor obtains a direct measurement of said quantity, and; a calibration device, having a processor, in communication with said direct sensor and said indirect sensor wherein said calibration device communicates a calibration algorithm to correct said indirect sensor.
 7. The dynamic calibration apparatus of claim 6 wherein said indirect sensor is contained within an instrumented sensor subsystem.
 8. The dynamic calibration apparatus of claim 6 wherein said calibration device sets an error limit and prevents calibration if the difference between said direct unit and said indirect measurement exceeds said error limit.
 9. The dynamic calibration apparatus of claim 6 wherein said primary indirect measurement is bounded by a finite time interval.
 10. The dynamic calibration apparatus of claim 6 wherein said secondary indirect measurement comprises one of power draw on an electric motor and current draw on an electric motor.
 11. The dynamic calibration apparatus of claim 6 wherein said primary indirect measurement and said direct measurement comprise one of torque, mud flow, and pressure. 